1. Field of the Invention
The present invention relates to a protection device for an assembled battery having a function of correcting dispersion of voltage among battery cells and an assembled battery system containing the same.
2. Description of the Related Art
In recent years, an electric vehicle and a hybrid vehicle have been attracting public attention in viewpoints of reduction of environmental load. A vehicle mounted battery which determines the traveling performance of the electric vehicle and the hybrid vehicle is demanded to have a high voltage of more than several tens of volts to enable the motor to be driven. For the reason, as the vehicle mounted battery, an assembled battery in which a plurality of battery cells each having a voltage of about 2 V are connected in series is used.
For the assembled battery, dispersion among the battery cells is problematic. Although the same current flows in each battery cell because the battery cells are connected in series, the voltages are different if the capacity is dispersed. If the voltages of the battery cells are different, there is a battery cell whose voltage reaches an upper voltage limit and other battery cell whose voltage drops to a lower voltage limit when it is discharged. Because the battery cell whose voltage reaches the upper voltage limit is in an over-charged state and the battery cell whose voltage reaches the lower voltage limit is in an over-discharged state, deterioration of battery performance is induced. To prevent the over-charge and over-discharge of the battery cell, the dispersion of voltage among the battery cells needs to be corrected.
JP-A H11-055866 (KOKAI) has disclosed an art for correcting the dispersion of voltage among the battery cells. According to JP-A H11-055866 (KOKAI), a voltage detection circuit is possessed in the battery so as to detect the voltage of each battery cell. By discharging charges of the battery cell whose voltage is higher than an average voltage via a switch and a resistor according to a result of this voltage detection, the voltages are adjusted to that average. By adjusting the voltage of each cell battery to the average, the dispersion of the voltage is corrected.
Although JP-A 2001-201522 (KOKAI) has disclosed a flying capacitor voltage detection circuit for the assembled battery, JP-A 2001-201522 (KOKAI) does not consider correction of the dispersion of voltages.
In JP-A H11-055866 (KOKAI), a dispersion correcting circuit having a switch for connecting a resistor and both ends of the battery cell to a resistor is required as well as the voltage detection circuit, thereby increasing the total quantity of devices. Because the increase of the devices leads to increases in area and cost if the protection devices incorporating the voltage detection circuit and the dispersion correcting circuit are integrated as for example, a semiconductor IC, it is desirable to use as small a quantity of the devices as possible.